Instead of using the footline layer, you could add the content of the footline to another layer which is behind the normal text. For example if your theme does not use the left sidebar, you could do:
\documentclass{beamer}
\usetheme{Madrid}
\setbeamertemplate{footline}{}
\setbeamertemplate{navigation symbols}{}
\makeatletter
\setbeamertemplate{sidebar canvas left}
{%
\tiny%
\leavevmode%
\hbox{%
\rule{0pt}{.99\paperheight}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
\usebeamerfont{author in head/foot}\insertshortauthor\expandafter\ifblank\expandafter{\beamer@shortinstitute}{}{~~(\insertshortinstitute)}
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
\usebeamerfont{title in head/foot}\insertshorttitle
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
\insertframenumber{} / \inserttotalframenumber\hspace*{2ex}
\end{beamercolorbox}%
}%
\vskip0pt%
}
\makeatother
\begin{document}
\begin{frame}
\color{red}
x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline x\newline
\end{frame}
\end{document}

(see How does beamer build the frame? for more available layers)
\begin{frame}[plain]{...} ... \end{frame}
for a too large contents. Makes more sense than overwriting the footline